1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus including a latent electrostatic image forming unit, an image bearing member, and a developing unit; and to a process cartridge.
2. Description of the Related Art
Conventionally, electrophotographic image forming methods include forming a latent electrostatic image on an image bearing member which has a photoconductive layer containing, for example, a photoconductive material; and depositing charged toner particles on the latent electrostatic image to form a visible image. After transferred onto a recording medium such as paper, the visible image is fixed by, for example, heat, pressure and solvent gas, to thereby obtain an output image.
In terms of a method for charging a toner to be used for forming a visible image, such electrophotographic image forming methods are roughly classified into two-component developing methods in which a toner and a carrier are stirred/mixed to charge the toner with friction generated therebetween; and one-component developing methods in which a toner is charged with no use of a carrier. Further, based on whether or not a magnetic force is utilized for retaining a toner on a developing roller, the one-component developing methods are further classified into one-component magnetic developing methods and one-component non-magnetic developing methods.
Hitherto, in copiers, complex machines based upon the copiers, and the like for which high-speed processing capability and favorable image reproducibility are required, the two-component developing methods have been employed in many cases due to demands for stable chargeability of toner particles, stable charge rising properties of the toner particles, long-term stability of image quality, etc.; whereas in compact printers, facsimiles, etc. for which space saving, cost reduction and the like are required, the one-component developing methods have been employed in many cases.
Also, nowadays in particular, colorization of output images is progressing, and demands for increase in the quality of images and stabilization of image quality are increasing like never before. For higher image quality, toners have been made smaller in average particle diameter, and particles of the toners have been made rounder in shape with their angular parts removed.
Generally, in an image forming apparatus which operates in accordance with any such electrophotographic image forming method, regardless of which developing method is employed, a drum-shaped or belt-shaped image bearing member is uniformly charged while being rotated, a latent image pattern is formed on the image bearing member by laser light or the like, and the latent image pattern is visualized as a toner image by a developing device and transferred onto a recording medium. After the toner image has been transferred onto the recording medium, untransferred toner components remain on the image bearing member. If such residues are directly conveyed to a place for the charging step, it often hinders the image bearing member from being uniformly charged; accordingly, in general, the toner components, etc. remaining on the image bearing member are removed at a cleaning step subsequent to the transfer step, thereby bringing the surface of the image bearing member into a clean enough state, and then charging is carried out.
Hitherto, in order for the latent image on the image bearing member to be uniformly developed for forming a visible image, a toner or developer is supplied in a temporally and spatially uniform amount to a developing region which is a developing gap spaced as uniformly as possible (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 09-211975 and 2000-194191).
Also, when an image bearing member before latent electrostatic image formation is ununiform in charged potential from place to place, the formed latent image formed thereon is adversely affected to be ununiform in potential, finally causing ununiformity of the formed visible image in some cases. Thus, when such an image forming method is employed, it is required that the image bearing member do not has charged potential ununiform from place to place.
Meanwhile, in general, a photoconductive layer used in the image bearing member is roughly classified into an inorganic photoconductive layer made, for example, of amorphous silicon and selenium; and an organic photoconductive layer made, for example, of polysilane and phthalopolymethine.
On the market, image forming apparatuses each having an image bearing member containing an organic photoconductive layer are often used in terms of safety of the material itself, allowing easy production, and cost.
The image bearing member containing an organic photoconductive layer is generally formed as follows. Specifically, a plurality of layers having different functions are sequentially formed on the surface of a base such as a conductive cylinder by repeating coating and drying at times required.
Thus, in order to uniform the charged potential, the photoconductive layer is required to have no thickness deviation. In particular, the thickness deviation of a charge transport layer having the dielectric function and contributing to maintenance of charges must be small sufficiently.
However, in an actual production of the image bearing member containing an organic photoconductive layer, much time and effort is required for forming a photoconductive layer having a uniform thickness, since the plurality of layers are formed on the surface of the base as described above. This causes elevation of the production cost for the image bearing member.
JP-A No. 09-211975 discloses a development sleeve having improved shape, in order to respond to a change in a developing gap which is caused when a developer enters the developing gap during use. But, such a development sleeve cannot compensate for the thickness deviation of the organic photoconductive layer to stabilize image quality. This proposal poses a problem in that it is difficult to provide a high-quality image at low cost.
JP-A No. 2000-194191 discloses a developing device having an inverted-crown-shaped developer bearing member. In this developing device, when the developer bearing member is pressed by a supply roller, a developing gap is constant between the developer bearing member and the image bearing member. But, such a developing device cannot compensate for the thickness deviation of the organic photoconductive layer to stabilize image quality. This proposal also poses a problem in that it is difficult to provide a high-quality image at low cost.
Further, JP-A No. 2006-98601 discloses a developing device in which a gap between a developer-controlling member and a developer bearing member can be controlled so that the amount of a developer supplied on the developer bearing member can be adjusted. This proposal can compensate for the difference in image density between the left-hand and right-hand sides. But, since the amount of a developer is made to be different between the left-hand and right-hand sides on the developer bearing member, unfavorable phenomena such as background smear tend to arise on the photoconductor. Also, this poses a problem in that extra efforts are required to adjust the amount of a developer supplied.